Automatic self-test system for a digital multiplexed telecommunication system

Abstract

An automatic self-test apparatus for use in a digital multiplexed communication system and self-testing idle circuitry coupled to idle telephone lines. The apparatus utilizes a tone generator which generates digital words describing an analog test signal and a detector which detects digital data on the internal data bus which describes this self-test analog signal. The data from the tone generator is coupled to the circuitry connected to the idle line through a time division multiplexer under the control of a controller consisting of a programmed microprocessor. The microprocessor controls when the transfer is made and to which telephone line it is directed. This control is accomplished by writing a control word to a multiplexer naming the source and destination for the transfer and the time slot in which it is to occur. When the circuitry connected to the idle line functions properly, the controller proceeds to the next idle line. If a malfunction is detected, the controller sets an alarm.

What is claimed is: 
     
       1. A maintenance circuit for a digital multiplexed communication system having a central office terminal and a remote terminal with a plurality of digital-to-analog and analog-to-digital line card interface circuits including audio line drivers and receivers for individual telephone lines, said maintenance circuit located in said remote terminal to automatically test said line card interface circuits whenever they are idle, comprising: a data bus coupled to said line card interface circuits for carrying digital data;   a generator means coupled to said data bus for generating a plurality of words of digital test data;   a detector means coupled to said data bus for detecting test data and for generating an output signal; and   means to automatically, continuously determine which said line card interface circuits are idle and for causing said generator means to send said digital test data to said idle line card interface circuits and for causing said detector means to be coupled to said idle line card interface circuits to detect the presence of digital test data after it has been converted to analog form and then back to digital form by said idle line card interface circuits.   
     
     
       2. Apparatus in a terminal of a telephone system for testing a voice channel within said terminal of said telephone system carrying plural audio channels, each said channel having a line card interface circuit coupling said terminal to a telephone line each said line card interface circuit including a digital-to-analog converter for carrying data toward said telephone line, and an analog-to-digital converter for carrying data toward said terminal and audio circuitry for the analog side of said interface circuit, comprising: means for periodically, automatically monitoring each of said plural audio channels to select an audio channel which is not in use;   means for generating a test signal and for connecting said test signal to said line card interface circuit so that said test signal propagates through said digital-to-analog converter, said audio circuitry and said analog-to-digital converter of said audio channel which is not in use; and   means for detecting whether said line card interface circuit which is not in use is operative by detecting the presence of said test signal at the output of said analog-to-digital converter.   
     
     
       3. An apparatus, as defined in claim 2, further comprising means responsive to said detecting means for indicating when a failure has occurred. 
     
     
       4. An apparatus, as defined in claim 2, further comprising means responsive to said detecting means for indicating when and on which audio channel a failure has occurred. 
     
     
       5. An apparatus, as defined in claim 2, wherein said telephone system includes a digital time division multiplexer and said generator means generates a plurality of digital words which, in combination, describe an analog test signal. 
     
     
       6. An apparatus, as defined in claim 5, wherein said plurality of digital words are directed by said monitoring means to said selected audio channel through said multiplexer and said plurality of digital words are looped through said selected audio channel and back through said multiplexer to said detector means. 
     
     
       7. An apparatus, as defined in claims 5 or 6, wherein all of said plural audio channels are connected to said multiplexer by one bus. 
     
     
       8. A self-diagnosing telephone terminal for use in a digital multiplexed span line telephone system for coupling a plurality of telephone lines to said span line, comprising: a digital-to-analog converter for coupling digital signals from said span line to a telephone;   an analog-to-digital converter for coupling analog signals from said telephone to said span line;   means for sensing whether said telephone is on hook;   audio signal processing means for driving analog signals from said digital-to-analog converter onto said telephone lines and for receiving analog signals from said telephone lines and for driving them into the input of said analog-to-digital converter;   means responsive to said sensing means for periodically, automatically injecting a digital test signal into said digital-to-analog converter when said telephone is on hook;   means for looping output signals from said digital-to-analog converter to the input of said analog-to-digital converter through said audio signal processing means; and   means responsive to said sensing means for detecting said digital test signals from the output of said analog-to-digital converter.   
     
     
       9. An apparatus, as defined in claim 8, further comprising a means responsive to said detecting means for indicating when said digital test signals are not present at the output of said analog-to-digital converter. 
     
     
       10. An apparatus, as defined in claim 8, further comprising a time division multiplexer means coupled between said digital-to-analog and analog-to-digital converters and said span line by a data bus and connected to said means for sensing and to said means for injecting, said multiplexer means for transferring said digital test signal from said means for injecting to the digital-to-analog converter coupled to said on-hook telephone selected by said means for sensing and for transferring said digital test signals from said analog-to-digital converter back to said means for detecting said transfers under the automatic control of said means for sensing. 
     
     
       11. Apparatus for testing an audio channel within a terminal of a telephone system in which plural audio channels share a common span line, where said system includes at least two terminals connected by said common span line, comprising: plural line termination means including per line audio amplifier circuits for connecting said terminal with plural telephone lines, said line termination means superimposing said plural audio channels of said plural telephone lines on said common span line;   a maintenance circuit comprising: means for sensing which of said plural telephone lines is not in use, for automatically generating a test signal, and for coupling said test signal to line termination means of said telephone line which is not in use;   means in said line termination means for looping said test signal back to said maintenance circuit through said per line audio amplifier circuits; and   means in said maintenance circuit for detecting said test signal.     
     
     
       12. An apparatus, as defined in claim 11, further comprising means responsive to said detecting means for ascertaining if said line termination means is operative and indicating the status of said termination means. 
     
     
       13. An apparatus, as defined in claim 11, wherein said line termination means includes a time division multiplexer. 
     
     
       14. An apparatus, as defined in claim 11, wherein said telephone system is a digital time division multiplexed system and wherein said line termination means converts analog signals from said plural audio channels into digital signals and multiplexes them through a time division multiplexer onto said common span line and demultiplexes digital signals from said span line and converts them to audio signals and couples them to selected ones of said audio channels and wherein said means for sensing generates a digital test signal and causes said line termination means to couple said digital test signal to the idle telephone lines automatically, periodically selected by said means for sensing.   
     
     
       15. Apparatus for testing a voice channel within a terminal of a multiplexed digital telephone system, where said system includes at least two terminals connected by a span line, comprising: plural line termination circuits for connecting said terminal with plural telephone lines, said circuits converting between analog signals on said telephone lines and digital signals in said terminal and including per line audio amplifiers to drive said telephone lines with analog signals and receive analog signals from said telephone lines;   means for multiplexing between digital signals in said plural line termination circuits and said span line to provide simultaneous transmission on said span line for a selected group of said plural telephone lines;   test circuit comprising: means for sensing which of said plural telephone lines is not in use, for automatically generating a digital test signal, and for coupling said test signal through said multiplexing means to the line termination circuit of said telephone line which is not in use;   means in said line termination circuits for looping said test signal through said per line audio amplifiers and back through said multiplexing means to said test circuit; and   means in said test circuit for detecting said test signal.     
     
     
       16. An apparatus, as defined in claim 15, further comprising an alarm means responsive to said detector means for indicating when no test signal has been detected for a selected telephone line not in use. 
     
     
       17. An apparatus for self-testing a digital multiplexed communication system, comprising: a data bus for carrying digital data describing an analog waveform;   a plurality of line interface means coupled to said data bus, said plurality of line interface means including a plurality of per line A/D and D/A conversion circuits and a plurality of audio line driver and line receiver amplifiers coupling a plurality of telephone lines to said data bus;   a test signal generator coupled to said data bus, generating a plurality of digital test words, and placing said digital test words on said data bus;   a detector coupled to said data bus, detecting said digital test words on said data bus;   a multiplexer coupled to said data bus, transferring said digital test words from said test signal generator to a selected one of said plural line interface means, and transferring said digital test words from said selected one of said line interface means to said detector on said data bus during selected time slots; and   a controller coupled to said test signal generator, said detector, said multiplexer and said plural line interface means, said controller for selecting the line interface means for said transfer, and to cause said multiplexer to transfer said digital test words from said test signal generator to a selected line interface means where said digital test signals propagate through said per line conversion and audio amplifier circuits and to transfer said digital test signal from said selected line interface means to said detector.   
     
     
       18. An apparatus, as defined in claim 17, wherein said plurality of line interface means includes means for converting said digital test words to an analog signal, looping said analog signal through the audio channel of said line interface means and reconverting said analog test signal to digital words for transfer back to said detector. 
     
     
       19. A maintenance circuit for a digital multiplexed communication system coupling a plurality of analog telephone lines coupled to telephones to a digital data transmission line, comprising: line interface means having a digital-to-analog converter and an analog-to-digital converter and audio interface circuits for each telephone line to interface said analog telephone lines to said data bus and for indicating when said telephones coupled to said telephone lines are on-hook and not in use;   a PCM data bus for carrying digital representations of analog waveforms;   a toner generator coupled to said PCM data bus for generating a plurality of words of digital test data which, in combination, describe an analog test signal and for placing said plurality of digital words on said data bus;   a detector coupled to said PCM data bus for detecting said test signal and for generating an output signal when said test signal is present;   an alarm circuit coupled to said main data bus;   a multiplexer having a line receiver coupled to a serial data input for converting serial format input data on said digital data transmission line to parallel format data for outputting on said PCM data bus and having a line transmitter coupled to a serial data output for converting parallel format data from said PCM data bus to serial format data for outputting on said digital data transmission line, said multiplexer coupled to said line interface means, said tone generator, and said detector by said PCM data bus, and having a control word input coupled to said main data bus for receiving a digital control word, said multiplexer for establishing a plurality of time slots and for establishing a data link for each time slot via said PCM data bus between one of said line interface means and said line receiver and said line transmitter or for establishing a data link between said detector and said tone generator and one of said line interface means, the selection of circuits to be linked and assignment of time slots controlled by said digital control word data; and   a controller coupled to said main data bus, and having address and control lines coupled to said multiplexer, said controller for generating a control word for each time slot established by said multiplexer, said controller also coupled to said line interface means by said data, address and control lines, said controller programmed to test each line interface means to determine which telephones are on-hook and not in use and to generate a control word for a self-test time slot, said control word causing said multiplexer to transfer said digital test data from said tone generator to said line interface means for any telephone line not in use and to cause said multiplexer to transfer digital data from said line interface means, which data has been converted to an analog test signal, propagated through said audio interface circuits and reconverted to digital test data, to said detector during the assigned time slot, said main data bus, address and control lines of said controller also coupled to said detector for controlling when said detector is enabled, and said controller programmed to read said output signal from said detector to determine when said line interface circuitry is functioning properly, and to cause said alarm circuit to indicate a failure on the tested line when no test signal is detected and said controller programmed to continuously so test idle line interface means.   
     
     
       20. An apparatus, as defined in claim 19, wherein said detector includes: a time slot counter coupled to said clock and timing bus from said multiplexer for counting the number of the time slot which said multiplexer is in at any time and for generating a binary representation thereof;   a time slot register coupled to said control, address and data lines from said controller for storing the binary representation of the time slot number assigned by said controller for self-testing of idle line interface circuits;   a comparator coupled to the binary outputs of said time slot counter and said time slot register for comparing the binary numbers appearing at each output and for generating an output signal indicating when there is a match;   a from/to control circuit coupled to the output signal of the comparator and coupled to a data line from the output of the time slot register, said data line carrying a digital data bit which is set or cleared by a data line from the controller, and said from/to control circuit coupled to the clock and timing bus from the multiplexer, said from/to control circuit for controlling whether the detector loads data from the data bus when data is being transferred from the tone generator a line interface circuit or when data is being transferred to the detector from a line interface circuit and for generating an output signal so indicating; and   a first means coupled to the data bus and to the output signal of the from/to control circuit for loading data upon command from the from/to control circuit from the data bus and detecting whether it describes an analog signal.   
     
     
       21. An apparatus, as defined in claim 20, wherein said time slot counter is comprised of: a NAND gate having an output and having its inputs coupled to the proper timing and control signals from the multiplexer such that said output changes state once for every time slot in which the multiplexer makes a data transfer;   a counter having its count input coupled to the output of said NAND gate and having its reset input coupled to the proper timing and control signal from the multiplexer such that the counter is incremented once for every transition at the output of the NAND gate but is reset to its starting count after a predetermined number of time slots; and   a latch having its data input coupled to the output of said counter and having its reset and clock inputs coupled to the proper timing and control signals from the multiplexer such that the latch is loaded from the counter once per time slot and reset after a predetermined number of time slots have passed.   
     
     
       22. An apparatus, as defined in claim 20, wherein said from/to control circuit includes: a NAND gate having an output for carrying said from/to signal and having a first input coupled to the output of said comparator and having a second input and having a third input coupled to the control signal of said multiplexer indicating when said multiplexer is loading said first data bus;   an inverter having its output coupled to said second input of said NAND gate; and   an exclusive-OR gate having its output coupled to the input of said inverter and having a first input coupled to said from/to bit at the output of said register, and having a second input coupled to a control signal from said multiplexer indicating when said multiplexer is writing to or reading from said first data bus.   
     
     
       23. An apparatus, as defined in claim 20, wherein said first means includes: a flip-flop having its set and reset inputs grounded and having its data input coupled to a bit on said first data bus and having its clock input coupled to the output signal of said NAND gate such that a bit from the first data bus is loaded upon a predetermined transition of said output signal;   a first resistor coupled to the output of the flip-flop;   a second resistor coupling ground to the opposite end of the first resistor thereby defining a node;   a first capacitor coupled to said node;   a second capacitor coupled to said node;   a third resistor coupled to the opposite ends of said first and second capacitors defining third and fourth nodes;   a first differential amplifier having its negative input coupled to said third node and its output coupled to said fourth node, thereby defining an active filter;   a fixed voltage reference source coupled to the positive input of said first differential amplifier;   a buffer amplifier coupled to the output of said first differential amplifier and having an output;   a third capacitor coupled between the output of the buffer amplifier and ground; and   a second differential amplifier having its positive input coupled to the output of said buffer amplifier and having its negative input coupled to the fixed voltage reference such that a peak detector is formed, said second differential amplifier for generating an output signal indicating the presence of transitions of the bit of said first data bus said flip-flop is coupled to when said fixed reference voltage is set at a predetermined value.   
     
     
       24. An apparatus, as defined in claim 19, wherein said line interface means comprises: a buffer coupled to the said data bus and to said address bus and to said clock and timing bus for storing data to be transferred to the multiplexer and for storing data transferred from the multiplexer;   a digital-to-analog converter coupled to said buffer and to said clock and timing bus for converting the binary data in the buffer to an analog signal;   a receive filter coupled to the output of digital-to-analog converter for removing any quantization noise from the analog signal;   an amplifier coupled to the output of the receiver filter for amplifying the analog signal;   a transformer having a first winding coupled between the output of the amplifier and ground;   a second winding in said transformer;   an amplifier having its input coupled to said second winding for amplifying signals inductively coupled onto said second winding;   a transmit filter coupled to the output of amplifier for removing frequencies higher than one-half of the sampling frequency; and   an analog-to-digital converter coupled to the buffer and to the clock and timing bus and coupled to the output of transmit filter for converting the analog signal at the output of the transmit filter to digital data describing an analog signal and for loading said buffer upon a control signal from said multiplexer.   
     
     
       25. An apparatus, as defined in claim 19, further comprising: a digital data transmission line termination circuit coupled to said serial data input and serial data output of said multiplexer to convert the serial format data to digital data into a bipolar code suitable for transmission on a T1-type digital data transmission line and for converting bipolar digital data received on said digital data transmission line to digital data suitable for transmission on said data bus.   
     
     
       26. A maintenance circuit for a digital multiplexed communication system having a central office terminal and a remote terminal with a plurality of digital-to-analog and analog-to-digital line card interface circuits for individual telephone lines, said maintenance circuit located in said central office terminal to automatically, continuously test said line card interface circuits whenever they are idle, comprising: a data bus coupled to said line card interface circuits for carrying digital data;   a generator means coupled to said data bus for generating a plurality of words of digital test data;   a detector means coupled to said data bus for detecting test data and for generating an output signal; and   means to automatically, continuously determine which said line card interface circuits are idle and for causing said generator means to send digital test data to said idle line card interface circuits and for causing said detector means to be coupled to said idle line card interface circuits to detect the presence of said digital test data after it has been converted to analog form and then back to digital form by said idle line card interface circuits.   
     
     
       27. A self-testing digital multiplexed telephone system for coupling a plurality of telephone lines from a central office switching network to a plurality of remotely located telephone lines, comprising: first means for converting analog signals from said central office telephone lines to digital data for transmission to said remote telephone lines and for converting incoming digital data from said remote telephone lines into analog data for application to said telephone lines from said central office, and including audio circuitry for driving the analog signals in both directions;   second means for carrying digital data between said first means and said remote telephone lines;   third means for multiplexing digital data from said first means onto said second means and for demultiplexing data from said second means for coupling by said first means to said telephone lines from said central terminal; and   fourth means for continuously selecting idle telephone line channels in said first means and for applying a digital test signal to said idle channels through a self-test time slot in said third means, and for detecting a digital test signal after it has passed through said third means and said first means after being converted to an analog test signal, passed through said audio driver circuitry of said first means and reconverted to digital test signal.   
     
     
       28. An apparatus as defined in claim 27, further comprising: fifth means for converting digital data from said first means to analog data and applying it to said remote telephone lines through a line driver audio amplifier and for receiving analog data from said telephone lines through a line receiver audio amplifier and for converting it to digital data for transmission to said first means;   sixth means for multiplexing digital data from said fifth means onto said second means and for demultiplexing digital data from said second means for conversion to analog signals and coupling by said fifth means to said remote telephone lines; and   seventh means for continuously selecting idle telephone line channels in said fifth means and for applying a digital test signal to said idle channels through a self-test time slot in said sixth means, and for detecting a digital test signal after it has passed through said sixth means and after, in said fifth means, said digital test signal has been converted to an analog test signal, propagated through all audio amplifiers interfacing with the idle channel, and reconverted to a digital signal.   
     
     
       29. A self-testing multiplexed telephone system for carrying signals between a plurality of telephone lines from a central office switching network and a plurality of remote telephone lines, comprising: a first means including audio line driver and receiver amplifiers for converting analog signals from said telephone lines from said central office switching network to digital data and for converting digital data to analog signals to drive said central office telephone lines;   a second means for carrying digital data;   a third means including audio line driver and receiver amplifiers to convert digital data from said first means to analog signals to drive said remote telephone lines and to convert analog signals from said remote telephone lines to digital data;   a fourth means for multiplexing said digital data from said first means onto said second means and for demultiplexing data from said second means for use by said first means in driving said central office telephone lines;   a fifth means for demultiplexing data from said second means for use by said third means to drive said remote telephone lines and for multiplexing digital data from said third means onto said second means;   a sixth means for continuously selecting telephone lines from said central office which are not in use and for causing a test signal to be propagated through all the audio and conversion circuits of said first means interfacing with the idle lines only and for detecting said test signal after it has propagated through said first means to determine the operability of the interface circuits of said idle lines.   
     
     
       30. An apparatus as defined in claim 29, further comprising: seventh means for continuously selecting remote telephone lines which are not in use and for causing a test signal to be propagated through all the audio amplifiers and conversion circuits of said third means interfacing with the idle lines only and for detecting said test signal after it has propagated through said third means to determine the operability of the interface circuits of said idle lines.   
     
     
       31. An apparatus as defined in claim 1 or 2 or 26 or 27 or 28 or 29 or 30, further comprising: means for generating a test signal representing a fixed frequency analog signal; and   means for determining whether the test signal returned from the telephone line card interface circuitry has a predetermined frequency.   
     
     
       32. A method of self-testing a terminal in a digital multiplexed telephone system having a multiplexer and a plurality of line interface circuits each including an audio line driver and line receiver which couple said terminal to a plurality of telephone lines, comprising the steps of: (a) determining which of said telephone lines is idle;   (b) propagating a test signal through said multiplexer and line interface circuits, including the audio amplifier line drivers and receivers coupled to said idle telephone lines;   (c) detecting the amplitude of said test signal after it has passed through the multiplexer and line interface circuits corresponding to the idle telephone lines.   
     
     
       33. The method of claim 32 wherein the step of detecting the test signal includes the step of determining the frequency of the test signal after it has passed through the multiplexer and line interface circuits corresponding to said idle telephone lines. 
     
     
       34. The method of claim 32 or 33, further comprising the steps (a), (b), and (c) performed independently at both the remote terminal and the central office terminal ends of said telephone system.